This invention relates to optical fibers, and, more particularly, to the fabrication of a dispenser that holds the optical fiber and permits its rapid payout when needed.
Optical fibers are strands of glass fiber processed so that light transmitted therethrough is subject to total internal reflection. Glass optical fibers are typically lab fabricated by preparing a preform of glasses of two different optical indices of refraction, one inside the other, and processing the preform to a fiber. The optical fiber is coated with a polymer layer termed a buffer to protect the glass from scratching or other damage. As an example of the dimensions, in a typical configuration the diameter of the glass optical fiber is about 125 micrometers, and the diameter of the optical fiber plus the polymer buffer is about 250 micrometers (approximately 0.010 inches). (The combination of optical fiber and buffer is sometimes termed an "optical fiber cable". As used herein, the term "optical fiber" includes both the glass optical fiber and the combination of optical fiber and buffer, except where it is clear from the context that the term refers only to the glass component.)
For such very fine optical fibers, the handling of the optical fiber to avoid damage that might reduce its mechanical strength and/or light transmission properties becomes an important consideration. In one approach, the optical fiber is wound onto a cylindrical or tapered cylindrical bobbin (collectively termed herein a "tapered" cylindrical bobbin, even though the angle of the taper may be zero) with many turns adjacent to each other in a side-by-side fashion. After one layer is complete, another layer of optical fiber is laid on top of the first layer, and so on. A weak adhesive is typically applied to the layers of optical fiber to hold them in place. The final assembly of the bobbin and the wound layers of optical fiber is termed a dispenser, and the mass of wound optical fiber is termed the fiber pack. When the optical fiber is later to be used, the optical fiber is paid out from the dispenser in a direction generally parallel to the axis of the tapered cylinder.
The adhesive is an important component of the system. In its final form it must be sufficiently strong to maintain the physical integrity of the optical fiber pack, but sufficiently weak to permit the turns and layers of optical fiber to be removed from the optical fiber pack during payout, without damaging any portion of the optical fiber. In the most common approach, the adhesive is dissolved, dispersed, or suspended in a liquid and supplied in a fluid, flowable form for application. The adhesive can be applied prior to winding the optical fiber, concurrently with the winding, or subsequent to the winding of a layer of the optical fiber. Some of the solvent typically evaporates during deposition of the adhesive. After the entire fiber pack of many layers of optical fiber is formed, the adhesive is cured to remove the remaining solvent and harden the adhesive to its solid, cured state. Curing can be accomplished by any of several approaches, such as the addition of a catalyst, condensation, or heating.
In an alternative approach, an adhesive curable by ultraviolet radiation has been used. The adhesive is comprised of fully (100 percent) reactive, solventless material that is applied to a layer of optical fiber after it is wound onto the underlying optical fiber pack. The adhesive is cured with ultraviolet radiation. The next layer of optical fiber is wound overlying the cured adhesive, and the process is repeated as necessary to build up the fiber pack.
There are important drawbacks to each approach. One uses solvents that may damage the buffer of the optical fiber and are also potential sources of environmental pollution when evaporated. When solvent-dispersed adhesives are used, voids may remain in the fiber pack after curing, because the adhesive/fluid preparation is typically greater than 90 percent by volume of liquid. There may also be other types of optical fiber pack defects resulting from heating the dispenser during curing. For the case of the ultraviolet-curable adhesive, the hardened adhesive layer is not a good substrate for the winding of the next overlying optical fiber layer. The next optical fiber layer typically does not lie in a regular pattern between the turns of the prior layer, but instead may have various types of winding irregularities.
There is therefore a need for an improved approach to the preparation of optical fiber dispensers. The present invention fulfills this need, and further provides related advantages.